a) Field of the Invention
This invention relates to a surface light source device having a light-conducting member.
b) Description of the Prior Art
A conventional surface light source device with a light-conducting member 1 is shown in FIG. 1. The conventional device includes a light-conducting member consisting of transparent sheet material with a specific thickness and having a rectangular shape as seen from the top in the drawing, one or two linear light sources 2, such as cold-cathode tubes, mounted close to either a single or both ends 1a of the light-conducting member 1, a diffusion plate 3 disposed along the front surface (a light emitting side) 1b of the light-conducting member 1, and a reflection surface 4 disposed along the back surface 1c (an opposite side to the light emitting side 1b of the light-conducting member 1). Further, the back surface 1c of the light-conducting member 1 may have a multiplicity of tiny rough surface areas, each containing a multiplicity of dots 5a, as shown in FIG. 2. Also, the surface 1c may have a multiplicity of dots 5a, as shown in FIG. 3. FIGS. 2 and 3 each show a left half portion of the back surface 1c of the light-conducting member 1 as viewed from below in FIG. 1.
In such a surface light source device wherein a pair of light sources are provided as shown in FIG. 1, light emerging from a pair of the light sources 2 enters adjacent end surfaces 1a, 1a, undergoes total internal reflection and is transmitted through the light-conducting member 1 toward the center, respectively. As the light passes through the light-conducting member 1 by total internal reflection, the light is diffused by dots 5a, as shown in FIGS. 2 and 3. Part of the light is emitted from the surface 1b of the light-conducting member 1. The light passes through the diffusion plate 3 to form diffuse light. To provide a uniform distribution of light in the diffusion plate 3 after passing through the plate 3, the surface 1c of the light-conducting member 1 has an appropriate pattern of the tiny rough surface areas 5 as shown in FIG. 2, or an appropriate distribution of dots 5a.
It is important that such a surface light source device gives an enhanced brightness and also provides a uniform distribution of brightness.
To provide an enhanced brightness in the surface light source device, in one of its known forms as shown in FIG. 4, linear light sources 2 are provided close to each side of the light-conducting member 1.
However, the surface light source device of FIG. 4 with four pieces of light sources has a difficulty in providing a uniform distribution of brightness. It is impossible to obtain a uniform distribution of brightness simply by providing dots, as shown in FIGS. 2 and 3, on the surface 1c of the light-conducting member 1.
Also, in another of its known forms, the conventional surface light source device used with the light-conducting member, as shown in FIG. 5, is such that a light-conducting member 1 has a full thickness in a region close to the light sources 2, but gradually decrease with distance from the light sources 2. However, in such a surface light source device, because each side edge varies in shape and size, it is difficult to increase the number of the light sources. Thus, there is no known device of this type that has been used with four pieces of the light sources, and thus, it is impossible to increase the number of the light sources and thus enhance the brightness in the device.
In still another of its known forms, the conventional surface light source device, as shown in FIG. 6, is such that, to increase the brightness in the device, a pair of linear light sources 2, one above another, is contained in each end surface 1a of the light-conducting member 1. Compared to the brightness in the device having a single light source for each end, however, this surface light source device provides only 1.6-1.8 times as much brightness, thus indicating a low utilization of light emanating from the light sources 2. This is due the linear light sources, such as cold-cathode fluorescent tubes (discharge tubes), dissipating heat for mutual heating and resulting in a lowering of light source performance.